1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases.
2. Description of the Related Art
Exhaust gases from automotive engines and other internal combustion engines contain harmful components such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx). It exhaust gases containing these harmful components are emitted into the air without any treatment, pollution and environmental degradation will occur. Therefore, exhaust gases containing these harmful components are emitted into the air after purified by such a purifying apparatus as a catalyst for purifying exhaust gases.
The catalyst for purifying exhaust gases converts NO x, HC and CO contained in exhaust gases into innocuous nitrogen, carbon dioxide or water by catalyst metal.
It is known that HC conversion is strongly influenced by temperature and generally carried out at not less than 300xc2x0 C. When catalyst temperature is low, catalytic activity of catalyst metal is too low to convert HC in exhaust gases. The catalyst for purifying exhaust gases is heated by exhaust gases. This fact indicates that the catalyst soon after an engine starts has a low temperature. Besides, exhaust gases immediately after an engine starts contain a large amount of HC and the ratio of HC in the emission is large.
Accordingly, it is a critical problem to suppress HC emission from a catalyst when the catalyst has a low temperature.
This catalyst for purifying exhaust gases which suppresses HC emission can be produced, for example, by forming a catalyst carrier layer composed of alumina, etc. on the surface of a catalyst support and forming a HC conversion part for converting HC and a NOx conversion part for converting NOx and CO separately on the catalyst carrier layer. In this catalyst, there are two kinds of separation of the HC conversion part and the NOx conversion part: separation in an axial direction of the catalyst support and separation in a thickness direction of the catalyst carrier.
Examples of this catalyst are disclosed in Japanese Unexamined Patent Publication Nos.H11-253758, H11-104462, H7-213910, H6-142519, H11-210451, and H11-221466.
Japanese Unexamined Patent Publication No.H6-142519 discloses a hydrocarbon adsorptive catalyst which is provided with a first layer of ZSM-5 zeolite ion-exchanged with at least one metal of Cu and Pd, which are effective for hydrocarbon adsorption, on a monolithic carrier, a second layer of a powder based on activated ceria and/or alumina containing at least one metal of Pt and Pd as a catalytic component on the first layer, and a third layer containing Rh as a catalytic component on the second layer.
Japanese Unexamined Patent Publication No.H7-213910 discloses an exhaust gas purifying catalyst comprising an adsorbing catalyst formed of a catalyst carrier and a zeolite layer coated thereon, wherein a catalyst layer composed of a powder based on activated ceria and/or alumina containing at least one element of Pt, Pd and Rh as a catalytic component is provided on the zeolite layer.
Japanese Unexamined Patent Publication No.H11-104462 discloses a catalyst-adsorber for purifying exhaust gases, which is prepared by covering and loading an adsorbing layer composed of an adsorbent with hydrocarbon adsorbability on a monolithic carrier and covering and loading a catalyst layer composed of a catalyst material with purifying ability for harmful components in exhaust gases on the adsorbing layer, the catalyst layer having a thickness of 10 to 120 xcexcm.
Japanese unexamined Patent Publication No.H11-210451 discloses an exhaust gas purifying catalyst device formed by providing, in exhaust passages, a HC-adsorbing catalyst comprising a double-layered part constituted by a HC-adsorbent lower layer and a three-way catalyst upper layer, and a single-layered part constituted by a three-way catalyst layer alone and positioned on the downstream side of the exhaust emission flow.
Japanese Unexamined Patent Publication No.H11-221466 discloses a catalyst for purifying exhaust gases, wherein a catalyst containing zeolite is arranged in a front stage part to an exhaust gas flow, and a catalyst consisting of three catalyst layers is arranged in a rear stage part to the exhaust gas flow. Of the three catalyst layers, the first catalyst layer contains at least one component of platinum (Pt), palladium (Pd) and rhodium (Rh) and at least one component of alkali metals, alkaline earth metals, and rare earth elements, the second catalyst layer contains alumina and/or silica, and the third catalyst layer contains zeolite including a copper component and a cobalt component, and the first, second and third catalyst layers are overlaid in turn.
Japanese Unexamined Patent Publication No.H11-253758 discloses a catalyst for purifying exhaust gases comprising: a first catalytic layer containing one or more components of a platinum component, a palladium component and a rhodium component and one or more kinds of components of alkali metals, alkaline earth metals and rare earth elements; a second catalytic layer containing xcex2-zeolite, formed on the first catalytic layer; and a third catalytic layer which is composed mainly of zeolite containing a copper component and/or a cobalt component and formed on the second catalytic layer. In this case, the second catalytic layer structurally contains silica (SiO2).
Moreover, as evidenced by severer automotive emission limits, there is a demand for an improvement in HC purifying ability and exhaust gas purifying ability of a catalyst for purifying exhaust gases.
The present invention has been conceived in view of the aforementioned actual circumstances. It is an object of the present invention to provide a catalyst for purifying exhaust gases which exhibits effective purifying characteristics even in a low-temperature range such as immediately after an engine start, and high exhaust gas purifying ability.
The present inventors have studied on the structure of the catalyst for purifying exhaust gases in order to attain the above objects, and found that the above objects can be attained by loading catalyst metal additionally on a front stage part and/or a rear stage part of a catalyst metal-contained carrier layer to the exhaust gas flow.
A catalyst for purifying exhaust gases according to a first aspect of the present invention characteristically comprises: a catalyst support having tubular passages through which exhaust gases flow in an axial direction; a coating layer formed on a surface of the catalyst support and composed of zeolite, refractory inorganic oxide, and a first catalyst metal loaded on a surface of the refractory inorganic oxide; and a second catalyst metal loaded on a front stage part of the coating layer, which is an upstream end of the exhaust gas flow, and/or a rear stage part of the coating layer, which is a downstream end of the exhaust gas flow.
The catalyst for purifying exhaust gases according to this aspect of the present invention is excellent in HC purifying ability, since the second catalyst metal is loaded on the front stage part and/or the rear stage part.
Moreover, the present inventors have studied about HC conversion and exhaust gas purification of a catalyst for purifying exhaust gases and found that the above objects can be attained by employing xcex2-zeolite, which has a high HC-adsorbing ability, and a Cexe2x80x94Zrxe2x80x94Y composite oxide, which has a high oxygen storage ability, for a catalyst for purifying exhaust gases.
A catalyst for purifying exhaust gases according to a second aspect of the present invention characteristically comprises a heat-resistant catalyst support, and a catalytic coating layer composed of xcex2-zeolite, a Cexe2x80x94Zrxe2x80x94Y composite oxide, refractory inorganic oxide and a catalyst metal and formed on a surface of the catalyst support.
The catalyst for purifying exhaust gases according to this aspect of the present invention is excellent in HC purifying ability, since xcex2-zeolite adsorbs HC from low temperature to high temperature. Besides, the catalyst for purifying exhaust gases attains high oxygen storage ability and excellent exhaust gas purifying ability since the catalyst for purifying exhaust gases includes a composite oxide.
(The First Aspect of the Invention)
The catalyst for purifying exhaust gases according to the first aspect of the present invention comprises a catalyst support, a coating layer, and a second catalyst metal.
The catalyst support is a member having tubular passages through which exhaust gases pass in an axial direction. Since the catalyst support has a structure having tubular passages through which exhaust gases pass, the catalyst can have larger contact area with exhaust gases. As the contact area with exhaust gases is larger, catalyst metal of the catalyst and exhaust gases to be purified are brought in more contact and the exhaust gas purifying ability of the catalyst is improved. This type of catalyst employs, for example, a monolithic honeycomb catalyst support. The catalyst support can be formed of a conventional material such as cordierite and other heat-resistant ceramics and stainless steel and other heat-resistant metals.
The coating layer is formed on a surface of the catalyst support and composed of zeolite, refractory inorganic oxide, and a first catalyst metal loaded on a surface of the refractory inorganic oxide. Since the coating layer contains the first catalyst metal, the coating layer can purify exhaust gases.
Zeolite adsorbs HC at low temperature and releases the adsorbed HC at high temperature. The HC adsorbed by zeolite at low temperature and released from the zeolite at high temperature are contacted with a first or a second catalyst metal and purified. This zeolite is not particularly limited, as long as it can adsorb and release HC. Examples of the zeolite include ZSM-5, USY, xcex2-zeolite, silicalite and metallosilicate. It is more preferable to employ xcex2-zeolite, which has an excellent HC-adsorbing characteristics.
The zeolite content of the coating layer is preferably 20 to 80 wt % based on the entire coating layer. That is to say, when the coating layer contains 20 to 80 wt % of zeolite, HC contained in exhaust gases are sufficiently purified. If the zeolite content is less than 20 wt %, because of the small zeolite amount, the amount of HC adsorbed is decreased and the HC-purifying ability of the catalyst deteriorates. On the other hand, the zeolite content of more than 80 wt % has an adverse affect on catalytic activity and decreases the purifying ability of the catalyst. It is to be noted that the coating layer is composed of zeolite, refractory inorganic oxide and a first catalyst metal, and does not include a second catalyst metal.
The refractory inorganic oxide forms a catalyst carrier for a first catalyst metal. The refractory inorganic oxide is preferably alumina. Owing to high heat resistance and high chemical stability at high temperature, alumina does not react with the first catalyst metal or exhaust gases. This alumina is more preferably activated alumina (xcfx84-alumina).
The first catalyst metal is loaded on the refractory inorganic oxide. Because the first catalyst metal is loaded not on zeolite but on the refractory inorganic oxide, the first catalyst metal can exhibit its catalytic ability effectively. If the first catalyst metal is loaded on zeolite, HC adsorbed by zeolite make the first catalyst metal suffer from HC poisoning and the catalytic ability of the first catalyst metal deteriorates.
The first catalyst metal preferably comprises at least one element selected from a group consisting of Pt, Pd and Rh. Since the first catalyst metal comprises at least one element selected from a group consisting of Pt, Pd and Rh, the first catalyst metal can purify HC, CO and NOx.
The first catalyst metal content of the coating layer is preferably 1 to 10 wt % based on the entire coating layer. That is to say, when the coating layer contains 1 to 10 wt % of the first catalyst metal, exhaust gases can be sufficiently purified. If the first catalyst metal content is less than 1 wt %, the amount of the first catalyst metal is small and purification of exhaust gases is insufficient. On the other hand, with the first catalyst metal content of more than 10 wt %, an improvement in exhaust gas purifying effect is small with respect to an increase in the amount of the catalyst metal loaded. The first catalyst metal content is more preferably 3 to 8 wt %. The coating layer is composed of zeolite, refractory inorganic oxide and the first catalyst metal and does not include the second catalyst metal.
The second catalyst metal is loaded on the front stage part of the coating layer, which is an upstream end of the exhaust gas flow, and/or a rear stage part of the coating layer, which is a downstream end of the exhaust gas flow. Since the second catalyst metal is loaded on the coating layer, the catalyst attains improved HC-purifying ability.
specifically speaking, in the catalyst of the present invention HC contained in exhaust gases at low temperature are adsorbed by zeolite of the coating layer. When the second catalyst metal is loaded on the front stage part, the second catalyst metal speedily rises to catalytic activity temperature and purifies the HC adsorbed by zeolite. On the other hand, when the second catalyst metal is loaded on the rear stage part, the second catalyst metal purifies the HC released from zeolite at a high efficiency. A catalyst having the second catalyst metal both on the front stage part and the rear stage part exhibits both the effects.
The second catalyst metal serves to purify exhaust gases. This second catalyst metal preferably comprises at least one element selected from a group consisting of Pt, Pd and Rh. Since Pt, Pd and Rh exhibit three-way catalytic activity, the second catalyst metal constituted by the at least one element selected from a group consisting of Pt, Pd and Rh can purify HC, CO and NOx.
The first catalyst metal and the second catalyst metal can be either the same catalyst metal or different catalyst metals.
The second catalyst metal content is preferably 1 to 7 wt % based on the entire catalyst for purifying exhaust gases. If the second catalyst metal content is less than 1 wt %, the amount of the second catalyst metal loaded is small and the effect of purifying HC is decreased. On the other hand, if the second catalyst metal content is more than 7 wt %, an improvement in the exhaust gas purification effect is small with respect to an increase in the amount of the second catalyst metal loaded.
It is preferable that the axial length of the front stage part and the rear stage part are respectively one-third to one-tenth of that of the catalyst for purifying exhaust gases. If it is shorter than one-tenth of the axial length of the catalyst, the volume of each stage part used for purification is smaller and the effect of the second catalyst metal loaded cannot be exhibited. On the other hand, if it is longer than one-third of the axial length of the catalyst, the density of the second catalyst metal loaded is smaller and the purification effect cannot be sufficiently obtained.
The coating layer preferably comprises a HC-adsorbing layer composed of zeolite and formed on a surface of the catalyst support, and a catalyst-contained layer composed of refractory inorganic oxide and a first catalyst metal and formed on the HC-adsorbing layer.
Since the coating layer has a layered structure of the HC-adsorbing layer and the catalyst-contained layer, the catalyst for purifying exhaust gases attains improved purifying ability. That is to say, since the catalyst-contained layer is formed on the surface of the HC-adsorbing layer, when HC adsorbed by the HC-adsorbing layer are released from the HC-adsorbing layer, all of the HC are brought in contact with the catalyst-contained layer. At this time, the HC released from the HC-adsorbing layer are purified not only by the second catalyst metal but also by the first catalyst metal contained in the catalyst-contained layer.
The coating layer may include an additive contained in a catalyst carrier layer of a conventional catalyst for purifying exhaust gases. Examples of the additive include BaO and cerium oxide. The amount of the additive can be the same as those of the conventional catalyst for purifying exhaust gases.
The catalyst according to this aspect of the present invention can be produced by forming a coating layer on the surface of a catalyst support and then loading the second catalyst metal. The coating layer can be formed by preparing a slurry containing materials for the coating layer and coating the prepared slurry on the surface of a catalyst support and drying and calcining the slurry.
When the coating layer comprises a HC-adsorbing layer and a catalyst-contained layer, the coating layer can be produced by forming the HC-adsorbing layer on the catalyst support first and forming the catalyst-contained layer second.
Loading of the second catalyst metal can be made by impregnating a predetermined part of the coating layer with a solution of the second catalyst metal and then drying and calcining the part.
since the catalyst for purifying exhaust gases according to this aspect of the present invention has the second catalyst metal on the front stage part and/or the rear stage part, the catalyst has an advantage of exhibiting a high HC purifying ability.
(The Second Aspect of the Invention)
A catalyst for purifying exhaust gases according to the second aspect of the present invention comprises a catalyst support and a catalytic coating layer.
The catalyst support is a member with heat resistance. The catalyst support should have a heat resistance to the temperature of exhaust gases to be purified.
The catalyst support can be constituted by ordinary catalyst supports used for a conventional catalyst for purifying exhaust gases. As an example of the catalyst support, it is possible to employ a monolithic honeycomb support formed of a heat-resistant material such as cordierite and other ceramics and stainless steel and other heat-resistant metals.
The catalytic coating layer is a layer composed of xcex2-zeolite, a Cexe2x80x94Zrxe2x80x94Y composite oxide, refractory inorganic oxide and a catalyst metal and formed on the catalyst support.
xcex2-zeolite has high HC-adsorbing ability and excellent heat resistance. In a catalyst for purifying exhaust gases, xcex2-zeolite, like ordinary zeolite, adsorbs HC in exhaust gases at low temperature and releases the adsorbed HC at high temperature.
xcex2-zeolite has a higher adsorbed-HC releasing temperature than other kinds of zeolite. Owing to the high HC-releasing temperature, when the adsorbed HC are released from xcex2-zeolite, the temperature is high enough for the catalyst metal to exhibit catalytic activity. Therefore, the HC released from xcex2-zeolite can be purified by the catalyst metal efficiently. As a result, the catalyst according to this aspect of the present invention attains improved HC-purifying ability.
The Cexe2x80x94Zrxe2x80x94Y composite oxide is excellent in oxygen storage ability of adsorbing and releasing oxygen. Specifically speaking, in the case of cerium oxide added for a conventional catalyst for purifying exhaust gases, grain growth is observed at high temperature and oxygen storage ability deteriorates. However, in the case of a Cexe2x80x94Zrxe2x80x94Y composite oxide, grain growth is suppressed and accordingly oxygen storage ability is exhibited even at high temperature. As a result, the catalyst according to this aspect of the present invention attains high efficiency of purifying exhaust gases.
The Cexe2x80x94Zrxe2x80x94Y composite oxide can form a solid solution in part.
It is preferable that the Cexe2x80x94Zrxe2x80x94Y composite oxide has a molar ratio of Ce, Zr, and Y in the range of 30 to 70:30 to 70:1 to 10.
The production method of the Cexe2x80x94Zrxe2x80x94Y composite oxide can be, for example, as follows:
First respective solutions of predetermined amounts of cerium salt, zirconium salt and yttrium salt are prepared and mixed together, thereby precipitating a mixture of cerium, zirconium and yttrium. Next, the precipitate is calcined at a temperature of not less than 500xc2x0 C., thereby obtaining a Cexe2x80x94Zrxe2x80x94Y composite oxide.
The catalyst metal serves to purify exhaust gases. This catalyst metal preferably comprises at least one element selected from a group consisting of Pt, Pd and Rh. Since Pt, Pd and Rh have three-way catalytic characteristics, the catalyst metal can purify HC, co and NOx.
The refractory inorganic oxide forms a catalytic coating layer and at the same time holds the catalyst metal in the coating layer. The refractory inorganic oxide is preferably alumina, since alumina has a high heat resistance and high chemical stability at high temperature. The alumina is preferably activated alumina (xcfx84-alumina).
The catalytic coating layer preferably comprises the HC-adsorbing layer composed of xcex2-zeolite and formed on the surface of the catalyst support, and the catalyst-contained layer composed of the Cexe2x80x94Zrxe2x80x94Y composite oxide, the refractory inorganic oxide and the coating layer catalyst metal and formed on the HC-adsorbing layer. In this aspect of the present invention, it is preferable that the catalytic coating layer comprises the HC-adsorbing layer and the catalyst-contained layer, but the catalytic coating layer does not exclude a mixture of xcex2-zeolite, the Cexe2x80x94Zrxe2x80x94Y composite oxide, the catalyst metal and the refractory inorganic oxide.
When the catalytic coating layer has a layered structure of the HC-adsorbing layer and the catalyst-contained layer, the catalyst for purifying exhaust gases attains improved purifying ability. In this case, since the catalyst-contained layer is formed on the HC-adsorbing layer, when HC adsorbed by the HC-adsorbing layer are released from the HC-adsorbing layer, all the HC are brought in contact with the catalyst-contained layer. Therefore, the HC are purified by the catalyst metal contained in the catalyst-contained layer.
The catalyst metal is preferably loaded on the surface of the refractory inorganic oxide and/or the Cexe2x80x94Zrxe2x80x94Y composite oxide. By loading the catalyst metal on the surface of the refractory inorganic oxide and/or the Cexe2x80x94Zrxe2x80x94Y composite oxide, the catalytic activity of the catalyst can be suppreused from deteriorating. If the catalyst metal is loaded on xcex2-zeolite, the catalyst metal suffers from HC poisoning due to the HC adsorbed by xcex2-zeolite and the catalytic activity of the catalyst metal deteriorates.
The catalytic coating layer may include an additive which is used for a catalyst carrier layer of a conventional catalyst for purifying exhaust gases. BaO is an example of this additive.
In the catalyst according to this aspect of the present invention, the xcex2-zeolite content of the catalytic coating layer is preferably 20 to 80 wt % based on the entire catalytic coating layer. Owing to the xcex2-zeolite content of 20 to 80 wt %, the catalytic coating layer can adsorb HC and the HC purifying ability of the catalyst improves. If the xcex2-zeolite content is less than 20 wt %, owing to the small xcex2-zeolite amount, the amount of HC adsorbed decreases and the HC purifying ability of the catalyst deteriorates. On the other hand, the xcex2-zeolite content of more than 80 wt % has an adverse affect on catalytic activity. The xcex2-zeolite content is more preferably in the range from 40 to 60 wt %.
The Cexe2x80x94Zrxe2x80x94Y composite oxide content of the catalytic coating layer is preferably 5 to 50 wt % based on the entire catalytic coating layer. By having the composite oxide, oxygen concentration becomes stable at the time of purifying exhaust gases. If the composite oxide content is less than 5 wt %, the effect of the addition is too small to purify CO and NOx in exhaust gases sufficiently. On the other hand, if the composite oxide content is more than 50 wt %, the refractory inorganic oxide content becomes small and conversion efficiency lowers. The composite oxide content is more preferably in the range from 10 to 30 wt %.
The catalyst metal content of the catalytic coating layer is preferably 1 to 10 wt %, based on the entire catalytic coating layer. That is to say, when the catalyst metal content of the catalytic coating layer is 1 to 10 wt %, exhaust gases are sufficiently purified. If the catalyst metal content is less than 1 wt %, owing to the small catalyst metal amount, exhaust gas purification is insufficient. On the other hand, if the catalyst metal content is more than 10 wt %, the effect of improving exhaust gas purifying ability becomes smaller with respect to an increase in the catalyst metal content. The catalyst metal content is more preferably in the range from 3 to 8 wt %.
The catalyst for purifying exhaust gases according to this aspect of the present invention can be produced by forming the catalytic coating layer on the surface of the catalyst support. The catalytic coating layer can be formed by preparing a slurry containing materials for the catalytic coating layer, coating the slurry on the surface of the catalyst support, and drying and calcining the coating.
When the catalytic coating layer comprises the HC-adsorbing layer and the catalyst-contained layer, the catalyst can be produced by forming a HC-adsorbing layer on a catalyst support and then forming a catalyst-contained layer.
The catalyst for purifying exhaust gases according to this aspect of the present invention is excellent in exhaust gas purifying ability, since the catalyst has xcex2-zeolite, which has excellent HC-adsorbing ability, and a Cexe2x80x94Zrxe2x80x94Y composite oxide, which has excellent oxygen storage ability.
Now, advantages of the catalyst according to the first and second aspects of the present invention will be described:
The catalyst for purifying exhaust gases according to the first aspect of the present invention has an advantage of exhibiting high HC purifying ability while maintaining purifying ability as a catalyst for purifying exhaust gases, because of comprising a coating layer composed of zeolite with HC-adsorbing ability and a first catalyst metal, and a second catalyst metal loaded on the front stage part and/or the rear stage part of the coating layer.
Specifically speaking, in a catalyst for purifying exhaust gases in which a second catalyst metal is loaded on a rear stage part, HC adsorbed by zeolite at low temperature are released from zeolite at high temperature and purified by the second catalyst metal. On the other hand, in a catalyst for purifying exhaust gases in which a second catalyst is loaded on a front stage part, early ignition of the second catalyst metal makes the timing of ignition of a first catalyst metal early, and HC adsorbed by zeolite are purified by the first catalyst metal. As a result, the catalyst of the present invention exhibits an advantage of exhibiting high HC-purifying ability.
On the other hand, in the catalyst for purifying exhaust gases according to the second aspect of the present invention, a catalytic layer has xcex2-zeolite, which has excellent HC-adsorbing ability, and a Cexe2x80x94Zrxe2x80x94Y composite oxide, which has excellent oxygen storage ability. Therefore, the catalyst has an advantage of exhibiting high HC-purifying ability while maintaining purifying ability as a catalyst for purifying exhaust gases. Specifically, since xcex2-zeolite has high heat resistance and excellent HC-adsorbing ability, HC adsorption can be carried out until the temperature becomes high. As a result, the catalyst according to the second aspect of the present invention is particularly excellent HC-adsorbing ability.